Programming with ZooKeeper - A basic tutorial

Introduction

In this tutorial, we show simple implementations of barriers and
producer-consumer queues using ZooKeeper. We call the respective classes Barrier and Queue.
These examples assume that you have at least one ZooKeeper server running.

Both classes extend SyncPrimitive. In this way, we execute steps that are
common to all primitives in the constructor of SyncPrimitive. To keep the examples
simple, we create a ZooKeeper object the first time we instantiate either a barrier
object or a queue object, and we declare a static variable that is a reference
to this object. The subsequent instances of Barrier and Queue check whether a
ZooKeeper object exists. Alternatively, we could have the application creating a
ZooKeeper object and passing it to the constructor of Barrier and Queue.

We use the process() method to process notifications triggered due to watches.
In the following discussion, we present code that sets watches. A watch is internal
structure that enables ZooKeeper to notify a client of a change to a node. For example,
if a client is waiting for other clients to leave a barrier, then it can set a watch and
wait for modifications to a particular node, which can indicate that it is the end of the wait.
This point becomes clear once we go over the examples.

Barriers

A barrier is a primitive that enables a group of processes to synchronize the
beginning and the end of a computation. The general idea of this implementation
is to have a barrier node that serves the purpose of being a parent for individual
process nodes. Suppose that we call the barrier node "/b1". Each process "p" then
creates a node "/b1/p". Once enough processes have created their corresponding
nodes, joined processes can start the computation.

In this example, each process instantiates a Barrier object, and its constructor takes as parameters:

the address of a ZooKeeper server (e.g., "zoo1.foo.com:2181")

the path of the barrier node on ZooKeeper (e.g., "/b1")

the size of the group of processes

The constructor of Barrier passes the address of the Zookeeper server to the
constructor of the parent class. The parent class creates a ZooKeeper instance if
one does not exist. The constructor of Barrier then creates a
barrier node on ZooKeeper, which is the parent node of all process nodes, and
we call root (Note: This is not the ZooKeeper root "/").

To enter the barrier, a process calls enter(). The process creates a node under
the root to represent it, using its host name to form the node name. It then wait
until enough processes have entered the barrier. A process does it by checking
the number of children the root node has with "getChildren()", and waiting for
notifications in the case it does not have enough. To receive a notification when
there is a change to the root node, a process has to set a watch, and does it
through the call to "getChildren()". In the code, we have that "getChildren()"
has two parameters. The first one states the node to read from, and the second is
a boolean flag that enables the process to set a watch. In the code the flag is true.

Note that enter() throws both KeeperException and InterruptedException, so it is
the reponsability of the application to catch and handle such exceptions.

Once the computation is finished, a process calls leave() to leave the barrier.
First it deletes its corresponding node, and then it gets the children of the root
node. If there is at least one child, then it waits for a notification (obs: note
that the second parameter of the call to getChildren() is true, meaning that
ZooKeeper has to set a watch on the the root node). Upon reception of a notification,
it checks once more whether the root node has any child.

Producer-Consumer Queues

A producer-consumer queue is a distributed data estructure thata group of processes
use to generate and consume items. Producer processes create new elements and add
them to the queue. Consumer processes remove elements from the list, and process them.
In this implementation, the elements are simple integers. The queue is represented
by a root node, and to add an element to the queue, a producer process creates a new node,
a child of the root node.

The following excerpt of code corresponds to the constructor of the object. As
with Barrier objects, it first calls the constructor of the parent class, SyncPrimitive,
that creates a ZooKeeper object if one doesn't exist. It then verifies if the root
node of the queue exists, and creates if it doesn't.

A producer process calls "produce()" to add an element to the queue, and passes
an integer as an argument. To add an element to the queue, the method creates a
new node using "create()", and uses the SEQUENCE flag to instruct ZooKeeper to
append the value of the sequencer counter associated to the root node. In this way,
we impose a total order on the elements of the queue, thus guaranteeing that the
oldest element of the queue is the next one consumed.

To consume an element, a consumer process obtains the children of the root node,
reads the node with smallest counter value, and returns the element. Note that
if there is a conflict, then one of the two contending processes won't be able to
delete the node and the delete operation will throw an exception.

A call to getChildren() returns the list of children in lexicographic order.
As lexicographic order does not necessary follow the numerical order of the counter
values, we need to decide which element is the smallest. To decide which one has
the smallest counter value, we traverse the list, and remove the prefix "element"
from each one.